Instructions:How to redeem your Offer Code
Simply enter the offer code in the "Offer Code" Box on the Basket page of the Checkout. Click 'Add' and the page will refresh to show you the new discounted total. You can only use each Offer Code once.
If you want to try the 3 other codes (that I know of) to maybe diversify a little, here they are:
GCAP03ON (this is printed wrong in the voucher offer. Here it says GCAP04ON)